Spontaneous Rxns

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Spontaneous Rxns
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Thermodynamics

• The study of energy in general is thermodynamics
• there are three laws of thermodynamics
• The zeroth law of thermodynamics sets up
  thermodynamic equilibrium.
• If a hot object and a cold object come into
  contact eventually a thermo equilibrium will be
  established between them
• This is the basis of our understanding of
  calorimetry

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Thermodynamics

• The first law of thermodynamics states that the
  energy of the universe is constant.
• Euniverse DEsystem DEsurroundings
• So if the system loses energy the surroundings
  must be absorbing to maintain the conservation of
  the energy
• This is our basis of endothermic/ exothermic
  reactions and enthalpy

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Thermodynamics

• The second law of thermodynamics leads us to
  deciding if a reaction is natural (spontaneous)
  or unnatural (nonspontaneous).
• Uses two variables to identify the spontaneity of
  a reaction
• It uses enthalpy (DH) and entropy (DS) in
  combination to define spontaneity
• The second law is our focus this block

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Spontaneous Rxns

• Many chemical and physical processes release the
  kind of energy that can be used to do work
• Such as driving the pistons of an
  internal-combustion engine.
• The energy our bodies receive from the conversion
  of glucose into ATP
• If a surplus of heat is produced in a chem rxn it
  is called Gibbs free energy (?G)
• Free energy is usually usable energy and is
  available to do work

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Spontaneous Rxns

• Its easy to see how exothermic reactions produce
  this free energy.
• Free energy is the fire and light and heat we
  observe during an exothermic reaction

• The energy used by burning charcoal to cook food
  on the grill is free energy
• Electrical energy that is converted to heat
  energy in a hotplate is free energy
• The temp of the body is maintained at 37?C by
  free energy

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Spontaneous Rxns

• Free energy can be obtained from a chemical
  reaction only if the reaction is spontaneous
• Only spontaneous reactions occur naturally

• We can write a rxn eqn for the decomp-osition of
  CO2 CO2 ? C O2
• But experience tells us that CO2 doesnt
  naturally decompose

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Spontaneous Rxns

• C O2 naturally combines to form CO2,
• Any time something burns, we have C and O2
  reacting to form CO2, not the other away around
• So the world of rxn eqns can be divided into 2
  groups
• One group contains eqns representing rxns that
  actually occur naturally or are spontaneous
• Group two do not tend to occur naturally, or at
  least not efficiently

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Spontaneous Rxns

• Every chemical rxn fights to establish chemical
  equilibrium
• the state at which the forward and reverse rxns
  take place at the same rate is the chemical
  equilibrium.
• the products appear to be favored, while in
  actuality a small percentage of the product is
  reverting to reactants
• Spontaneous rxns are those rxns that favor the
  formation of products upon reaching equilibrium

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• In contrast, rxns that do not favor the formation
  of products at the specified conditions are not
  spontaneous
• Non-spontaneous rxns do not produce substantial
  amnts of products at equili.
• Think about the rxn

• The formation of CdS
  NaNO3 is the spontan-
  eous direction,
• CdS is insoluble yellow powder

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Spontaneous Rxns

• CdS isnt available to revert back into Cd(NO3)2,
  therefore, backward is the nonspontaneous
  direction
• Spontaneous nonspontaneous dont have anything
  to do with the rate of the rxn
• It deals only with whether or not the rxn is
  naturally occurring
• Some rxns that are spontaneous proceed so slowly
  that they appear to be nonspontaneous

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Spontaneous Rxns

• The rxn of sugar with oxygen, for exa-mple,
  produces carbon dioxide water

• Isnt a bowl of sugar on a table doing nothing?
• Might we assume that the equilibrium between
  sugar, O2, CO2, H2O greatly favors sugar O2?
• However, the favored direction is actually the
  products
• It just take thousands of years to come to
  completion

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• When you supply energy in the form of heat, the
  rxn is much faster, only then is it obvious that,
  at equil, the formation of CO2 H2O is highly
  favored
• Some rxns that are non-spontaneous under one set
  of conditions may be spontaneous under other
  conditions
• temp or press, for example, adjustments may
  determine whether or not a rxn will be
  spontaneous
• Photosynthesis is a non-spontaneous rxn and could
  not be driven to completion w/o the energy
  supplied by the sun

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• Sometimes a non-spontaneous rxn can be made to
  occur if it is coupled to a spontaneous rxn
• Coupled rxns are a common feature of the complex
  biological processes that take place in living
  organisms
• Within cells, a series of spontaneous rxns
  release the energy stored in glucose
• There are molecules (ATP ADP) in a cell that
  can capture and transfer free energy to
  non-spontaneous rxns.
• Assists in the the formation of proteins

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Entropy

• Recall that enthalpy changes or energy changes
  accompany most chem and phys processes?
• Combustion rxns release a large amount of heat
  energy so they are exothermic and obviously
  spontaneous
• Exothermic rxns which produce free energy are
  spontaneous,
• But what about endothermic rxns?
• Endothermic rxns absorb energy rather than
  produce it so how can they be energetically
  spontaneous?

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Entropy

• Consider this endothermic process
• ice absorbs energy in order to change phase from
  solid to liquid
• Considering only heat changes, the energy of the
  water is higher than the energy of the ice
• Yet ice obviously does melt
• So whats the deal?
• Rxns can also be spontaneous if the result of the
  rxn leans toward energy dispersal

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Entropy

• If you leave a piece of steel out in the air it
  will rust (you almost cant prevent it)
• If you punch a tiny hole in a balloon the air
  will escape through the hole.
• A hot object will eventually cool down.
• If you drop a rock it will fall down with a bang.
• If you smack a rock hard enough with a hammer it
  will shatter.
• Even the most advanced machine will eventually
  break down with repeated use

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Entropy

• All of those illustrations lead to a profound
  generalization in all everyday or exotic
  spontaneous physical or chemical happenings,
  energy flows from being localized or concentrated
  to becoming more spread out or dispersed.
• The measure of the dispersal of energy in a
  system is known as entropy (?S)
• ?S ?Sproducts - ?Sreactants
• Energy flows in the direction of hotter to cooler
  because that direction results in an increase of
  entropy.

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• Entropy is one of the most powerful laws in the
  universe
• In the past entropy has been described as a
  tendency to increasing chaos or increasing
  disorder
• ?S (J/molK) of a chemical rxn can be calculated

2H2 O2 ? 2H2O
130.7
205.1
188.3
S
?Srxn 2(188.3) 2(130.7)205.1
?Srxn -89.9 J/molK
(-) indicates an overall decrease in entropy
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ClassWork 1

1. What is the change in entropy for the following
   reaction and is there an increase or decrease in
   entropy overall?

2Al 6HCl ? 2AlCl3 3H2

1. The ?S of the following decom-position is 361.1
   J/molK. The entropy of H2 and N2 is 130.7 J/molK
   and 111.3 J/molK respectively. What is the
   entropy of NH3?

2NH3 ? N2 3H2
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Entropy

• Rxns or processes that move toward increased
  disorder or increased disper-sion of energy are
  favored entropically.
• Positive changes in entropy are favored
• We can look for an increased disorder to indicate
  a positive change in entropy
• entropy increases from solid to liquid to gas
• entropy increases in rxns in which solid
  reactants form liquid or gaseous products and
  liquid reactants to form gases
• Etc.

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Entropy

• Entropy increases when a substance is
  particulated
• Grinding,
  chipping,
  tearing,
  ripping,
  smashing,
  
  etc.

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Entropy

• Entropy increases in a chemical rxn in which the
  products are more numerous than the reactants
• Decomposition rxns are spontaneous in part
  because of their movement toward increasing less
  concentrated or localized energy.

2H2O ? 2H2 O2
?
2 particles ? 3 particles
2H2 O2 ? 2H2O
?
3 particles ? 2 particles
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Entropy increases in chemical reactions in which
the products are more numerous than the
reactants.
?
?
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Entropy

• Entropy tends to increase as temperature
  increases
• As the temp increases, the molecules move faster
  and faster, which decreases the localization of
  the energy of the system

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Entropy increases as a substance is
dissolved. Even a highly ordered crystal can be
pulled made more random
when pulled apart by water.
?
?
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Heat, Entropy, Free Energy

• Determining the spontaneity of a rxn is
  accomplished by examining both energy entropy
• An exothermic rxn combined by an increase in
  entropy or disorder, is identified as spontaneous
  since both factors are favorable
• For example in the combustion of carbon
• A combustion is exothermic so the energy is
  favorable

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Heat, Entropy, Free Energy

• C(s) is converted to CO2(g) so there is a
  favorable change in entropy
• Since both energy and entropy are favorable, the
  rxn is considered spontaneous
• The reverse rxn CO2 ? C O2 ? is non-spontaneous
• In this case neither the energy nor entropy is
  proceeding favorably
• A rxn may also be spontaneous if a decrease in
  entropy is offset by a large release of heat

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Heat, Entropy, Free Energy

• Or an endothermic rxn (unfavorable) may be
  spontaneous if an entropy increase offsets the
  heat absorption
• i.e. energy change entropy change work in
  opposition when ice melts
• Ice melting absorbs heat energy, which is
  endothermic, but is favorable entropy so the
  overall rxn is considered spontaneous
• Either of the 2 variables, but not both, may be
  unfavorable remain spontaneous

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Calculations with DG and DS

• The change in free energy of a system can be
  calculated by finding the difference between the
  change in enthalpy, and the product of the Kelvin
  temperature and the change in entropy.

DG DH - TDS

• This expression is for substances in their
  standard states.
• Each of the variables can have positive our
  negative values, which leads to four different
  combinations of terms.

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Calculations with DG and DS

• If DH is negative and DS is positive, then both
  terms on the right in the free energy equation
  are negative
• Both heat energy and entropy are contributing to
  the process being spontaneous
• If DH is positive (endothermic) and DS is
  negative (decrease in randomness)
• process is never negative so therefore the
  reaction can never be spontaneous

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Calculations with DG and DS

• DG must be negative in order for the rxn or
  process to be spontaneous
• For example in this rxn at room temp

C2H4(g) H2(g) ? C2H6(g)
SPONTANEOUS

• If
• DS -.1207kJ/molK (decrease in entropy)
• DH -136.9kJ/mol (exothermic)
• What is DG?

DG DH - TDS
DG(-136.9kJ/mol)(298K)(-.1207kJ/molK)
DG -101.1 kJ/mol
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• For the rxn NH4Cl(s) ? NH3(g) HCl(g), at
  298.15K, DH 176 kJ/mol and DS .285kJ/molK,
  Calculate DG, and tell whether this rxn can
  proceed in the forward direction at 298.15 K.

NOT SPONTANEOUS
DG DH - TDS
DG(176 kJ/mol)(298.15K)(.285kJ/molK)
DG 91.0 kJ/mol
Positive free energy means that at this
temperature this reaction does not occur
naturally.
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Relating Enthalpy, Entropy, and Free Energy Changes to Rxn Occurrence Relating Enthalpy, Entropy, and Free Energy Changes to Rxn Occurrence Relating Enthalpy, Entropy, and Free Energy Changes to Rxn Occurrence
DH DS DG
- value (exothermic) value (disordering) Always negative
- value (exothermic) -value (ordering) Negative at lower temps
value (endothermic) value (disordering) Negative at higher temps
value (endothermic) -value (ordering) Never negative
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ClassWork 2

1. Calculate ?G? at 25C given the following
   information and decide if the reaction is
   spontaneous

H2O(l) ? H2O(g) ?H? 44kJ/mol

1. What is the minimum temperature in celsius, of
   the above phase change, that will make the
   reaction spontaneous?